1. Field of the Invention
The present invention relates generally to production of nuclear fuel rods for use in nuclear reactors and, more particularly, is concerned with a unique automated system for the production of nuclear fuel rods starting from the conversion of a radioactive gas to powder, through the fabrication of the powder into pellets, to completion of the assembly of the fuel rods.
2. Description of the Prior Art
Conventional nuclear reactors include fuel elements, generally called fuel rods. The fuel rods contain fissile material and are grouped together in arrays which are organized to produce a neutron flux in the reactor core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A coolant such as water is pumped upwardly through the arrays of fuel rods in the reactor core in order to extract some of the heat for the production of useful work.
Typically, a fuel rod is composed of an elongated hollow metallic tube which contains the nuclear fuel material in the form of a stack of cylindrical fuel pellets. The tube is closed at its opposite ends by upper and lower end plugs which are rigidly attached to the tube ends by girth welds so as to hermetically seal the tube. During operation of the reactor core, the fuel rods are subjected to high temperatures and pressures within the core which cause elongation of the tube and pellets due to thermal growth and vibration of the fuel rods due to coolant flow. Thus, the pellets are fabricated to very exacting dimensions so as to produce a controlled diametrical clearance between the pellets and the inside of the tube to accommodate pellet growth due to thermal expansion and fuel swelling. Additionally, in view that pellets are brittle and will easily chip upon impact, a coil spring is ordinarily disposed within the tube between the upper end plug and the top of the pellet stack to restrain damaging impacts between the pellets due to rod vibration.
Fuel rod manufacturing conventionally involves beginning with the radioactive material in powder form and then blending it to the desired chemical composition. The properly blended powder is then made into pellets by first forming it into slugs, then granulating the slug and mixing a lubricant with the granulates, and lastly pressing the lubricated granulates into green pellets. The green pellets are fed into a sintering furnace where high temperatures sinter the pellets in a hydrogen atmosphere to achieve the required density and microstructure. After exiting the furnace, the sintered pellets are fed to a wet grinding process for grinding them to precise dimensions. Before insertion into the fuel rod tube, the finished pellets are visually inspected. After the pellets are placed in the tubes, the completed fuel rods are subjected to several different inspections.
Due to the fact that fissile material is involved, fuel rod manufacturing up to the present time has been carried out in conformity with the regulatory requirement of geometric control of the radioactive material being converted into fuel pellets. Geometric control relates to a safeguard which eliminates the possibility of a chain reaction occurring by limiting the quantity of radioactive material assembled together to an amount significantly less than the critical mass needed for fission. This safeguard was implemented by the performance of a high degree of manual handling of radioactive materials and fuel rod components during the various manufacturing stages of what has been termed a batch mode of operation.
For instance, a batch of radioactive material of a given enrichment had to be processed completely through a given stage of the manufacturing process and the equipment emptied of all residual material of that enrichment before material from a different batch having a different enrichment could be processed. Thus, with respect to each batch, typically, in the initial stage each worker carried an individual container filled with a small quantity of radioactive material from the batch in powder form to the blender. Once the material was properly blended, the worker then manually transferred the blended powder to the pelleting stage where this quantity of material was fabricated into pellets. The green pellets were then loaded manually by the worker into a sintering boat which was taken to the infeed end of a sintering furnace and then unloaded manually. After being conveyed through the furnace, the sintered pellets were manually picked up and fed to the wet grinding station. Then, the ground pellets were manually placed on inspection trays. After visual inspection, the pellets were manually inserted into tubes which in the meantime had been manually handled through various stages involving the inspection and cleaning of the tubes and attachment of end plugs thereto.
Another regulatory requirement which was implemented most effectively by the performance of a high degree of manual handling of radioactive materials and fuel rod components during the various stages of the manufacturing operation was the need for traceability of the radioactive material from its initial powder form to its final form as pellets in a completed fuel rod. Without much difficulty, a worker who began with a certain quantity of radioactive material from a known batch and transported it through successive stages of the manufacturing operation could identify which completed fuel rods contained material from the particular batch.
While the high degree of manual involvement in fuel rod manufacturing up to the present time has assisted the nuclear industry in meeting the regulatory requirements of geometric control and traceability and thus has served the industry well over the past several decades, such involvement has tended to constrain improvement in manufacturing productivity and product quality. Consequently, a need has evolved for a different approach to fuel rod manufacture which promises increased manufacturing efficiency and productivity and improved product quality and reliability while at the same time meets all regulatory requirements.